Born in 2014, the now-four-year-olds were also the first set of semi-identical (also known as sesquizygotic) twins to be identified during pregnancy, according to a study published in the New England Journal of Medicine this week.
While doctors originally thought that the twins were identical, an ultrasound at 14 weeks revealed that the babies were actually different genders, a unique phenomenon that’s impossible for identical twins.
Sharing 89 perecnt of their overall DNA, the Australian twins share 100 percent of their mother’s genes and just a portion of their father’s.
“Three sets of chromosomes are typically incompatible with life and embryos do not usually survive,” Dr. Michael Gabbett, one of the study’s lead researchers, said in an online statement. “In the case of the Brisbane sesquizygotic twins, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins.”
The first set of semi-identical twins was discovered in the United States in 2007. Because it’s such a rare condition, scientists spent two years combing through 968 cases of dizygotic twins to ensure there weren’t any prior cases that were misidentified.
“We needed to confirm the results with really exhaustive studies in multiple labs (in multiple states/countries), and with tests involving millions of genetic variants,” Dr. Nicholas Fisk, one of the specialists who cared for the mom and her twins, told CNN.
Dr. Fisk concluded, “We know this is an exceptional case of semi-identical twins. While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing,” FOX News reports.